This invention relates to a process for preparing N-phosphonomethylglycine by the oxidation of N-phosphonomethyliminodiacetic acid using a peroxide and a homogeneous catalyst system. More particularly, this invention relates to a process for producing N-phosphonomethylglycine by the peroxide oxidation of N-phosphonomethyliminodiacetic acid using a salt of a selected metal in the presence of a dipyridyl compound.
N-Phosphonomethylglycine, known in the agricultural chemical art as glyphosate, is a highly effective and commercially important phytotoxicant useful in controlling the growth of germinating seeds, emerging seedlings, maturing and established woody and herbaceous vegetation, and aquatic plants. N-Phosphonomethylglycine and its salts are conveniently applied in an aqueous formulation as a postemergent phytotoxicant for the control of numerous plant species. N-Phosphonomethylglycine and its salts are characterized by broad spectrum activity, i.e., the controlled growth of a wide variety of plants.
Numerous methods are known in the art for the oxidation of the N-phosphonomethyliminodiacetic acid to N-phosphonomethylglycine. For example, U.S. Pat. No. 4,952,723 to Fields, et al. discloses a process for the production of N-phosphonomethylglycine by contacting N-phosphonomethyliminodiacetic acid with a molecular oxygen-containing gas in the presence of a catalyst selected from the group consisting of the salts and salt complexes of manganese, cobalt, iron, nickel, chromium, ruthenium, aluminum, molybdenum, vanadium and cerium, and an effective amount of a dipyridyl compound. There is no suggestion, however, that peroxides could be used instead of the molecular oxygen-containing gas.
U.S. Pat. No. 3,954,848 to Franz discloses a process for the production of N-phosphonomethylglycine by reacting N-phosphonomethyiminodiacetic acid with an oxidizing agent, such as hydrogen peroxide, in an aqueous acidic medium in the presence of a strong acid at a temperature of from about 70.degree. C. to about 100.degree. C. It is disclosed that one should employ at least 2 moles of the hydrogen peroxide for each mole of the N-phosphonomethyliminodiacetic acid, and preferably more.
Hungarian Patent Application No. 187,347 discloses a process for the preparation of N-phosphonomethylglycine by the oxidation of N-phosphonomethyliminodiacetic acid with peroxides using a catalytic amount of a metal compound selected from compounds of silver, iron, tin, lead, manganese or molybdenum. Molybdates are preferred. At temperatures lower than 80.degree. C., usually a contaminated end product is obtained. Typically, the reaction is carried out at a temperature of above 80.degree. C. and preferably about 100.degree. C. at pressure exceeding atmospheric. It is further disclosed that at least two mole equivalents of peroxide should be used for each mole equivalent of N-phosphonomethyliminodiacetic acid.
Although satisfactory results are obtained by the above processes to make N-phosphonomethylglycine, all of them suffer from one or more disadvantages, such as the use of excessive amounts of peroxide, and/or reaction at elevated temperatures and pressures. Now, there is disclosed a process which provides N-phosphonomethylglycine in high yields at modest temperatures and at atmospheric pressure.